What program you use?..

Wall-to-Wall Write on Ollgabopi's Wall

so you wonder how you can put 2 characters into one screen, simple...
You just have to use the blue screen and green screen technique.
Oxhorn have some great tutorials that helps you to do master pieces :P
Watch them here:

http://www.youtube.com/watch?v=Vb5_preNI...

Hope I could be some help :)

Wall-to-Wall Write on calixworld2's Wall